Radio bomb release system



Aug. l5, 1950 D. G. c. LucK moro nous RELEASE lsafs'ma Filed Feb. 6, 1946 u INVENTOR.

Patented Aug. 15, 1950 2,518,916 immo BOMB RELEASE SYSTEM David G. C. Luck, Princeton, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application February 6, 1946, Serial No. 645,912

Claims.

This invention relates to radio bomb release systems and more particularly to improvements in systems of the time from target computing type, such as those described in copending U. S. patent application Serial No. 531,953, led April 20, 1944, by Royden C. Sanders, Jr., entitled Radio Bomb Release System, now matured to Patent No. 2,416,223. Such systems operate by measuring the range of a. selected target from the bomber, and the speed of the bomber with respect tothe target, computing the predicted time from target in response to said measurements. The time from target is defined as the period which will be required after a given instant for the bomber to reach a point directly over the target. At a predetermined time from target, depending upon the altitude of the bomber craft, the bomb release mechanism is energized.

Any system which measures the range and speed of a craft with respect to a target by means of radio signals reflected from said target will respond to the true, or slant range, rather than the projection of this distance along the line of flight of the craft. Likewise, the slant speed is measured instead of the speed component along the line of night. 'I'hus the quotient of the measured range and the measured speed is ordinarily an approximation to the true time from target, and one which becomes increasingly erroneous as the target is approached. The computed time from target may be corrected satisfactorily, within certain limits of slant speed, by using linear approximations to the relationships between slant range at release, slant speed at release, and altltude.

It is the principal object of the present invention to provide methods of and means for computing, automatically the time from target in accordance with its exact relationships to slant speed, slant range and altitude, rather than any approximations thereto.

Another object is to provide systems of the described type which automatically take into account the effects of vertical components in the flight of the bomber craft.

Still another object is to provide improved methods of and means for producing an adjustable range lead to enable dropping a bomb a predetermined distance ahead or behind the target.

The invention will be described with reference to the accompanying drawings, wherein:

Figure 1 is a geometrical diagram illustrating the elements of a bomb release problem,

Figure 2 is a schematic block diagram of an embodiment of the present invention,

Figure 3 is a schematic diagram of a modificaf tion of the system of Figure 1,

Figure 4 is a graph illustrating variations in frequency of signals transmitted and received in the operation of the system of Figure 2, and

Figure 5 is a schematic diagram of a further modification of the system of Figure 2.

Refer to Figure 1. It is assumed that a bomber, at present at the point l, is flying toward a point 3 at a distance D from the point I, and at an a1- titude H directly over a target at the point 5, at a uniform ground speed G with respect to the point 3. The time from target, i. e. the time which will be required for the craft to reach a point directly over the target, is:

As mentioned before, the distance D and ground speed (horizontal component of speed) G are not measured directly by radio equipment on the bomber, However, the slant range R, the slant speed S, the altitude H, and the vertical speed V can be measured, as described in detail hereinafter.

(2) H=R sin a and (3) =R cos a where a is the target depression angle. The slant speed is:

(4) S=G cos +V sin a where V is the vertical component of the speed of the bomber craft taken as positive when directed downward. If the bomber is flying horizontally, V is zero. Eliminating the quantities D, G, and a from Equations 1, 2, 3 and 4, the time from target T is found to be given in terms of the measured quantities R, S, H and V by:

Neglecting the effect of air resistance, as is permissible for moderate altitudes and speeds, the time of fall T: of a bomb is related to its altitude H and the Vertical speed V at the instant of its release as follows:

Since the forward velocity of the bomb is the same as that of the aircraft, the release xr' comes: 4

eea-ey and (6) becomes:

In the embodiment of the present invention illustrated in Figure 2, the bomber carries a range measuring device II and a speed measuring device I3. The device II may be similar to the well known frequency modulation type radio altimeter, but arranged to transmit and receive signais in a generally forward, rather than downward direction. 'Ihe device II provides an electrical output having a magnitude (e. g. current or voltage) proportional to the slant range R of a. target toward which it is directed. The speed measuring device also'may be of the radio reflection type, utilizing the Doppler effect on the signals transmitted to and received from the target to provide an electrical output porportional to the slant speed S of the bomber relative to the target.

The range measuring device II is connected to a sensitivity control I5, which serves to adjust the proportionality of the electrical output to the range. More specifically, the output of the device I I is a voltage proportional to the range, i. e. a certain number of volts represents a certain number of feet. The control I modifies this output to make a different number of volts represent a certain number of feet. rIhis modified output is applied to one pair of input terminals of a wattmeter mechanism I1. It is also applied, together with and in opposition to the output of the speed measuring device I3, toI the other pair of input terminals of the wattmeter I'I.

The output shaft of the wattmeter I1 is coupled mechanically by a shaft I9 to a biassing device such as a torque motor 2|. The torque motor ZI may be an ordinary direct-current meter, and is provided with torque control means 23 for regulating the input to the motor 2l and hence the torque developed by it. The connections to the motor 2l are such that its torque opposes that of the wattmeter I'I.

A pair of contacts 25 is coupled to the shaft I9, in such manner that the torque of the wattmeter I1 tends to hold it open, and is adjusted so as to close when the total torque is zero, i. e. that of the wattmeter I'I equals that of the torque motor 2 I.

`The contacts 25 are connected to the actuating circuit of a bomb release mechanism, not shown.

The input to the lower terminals of the wattmeter I'I is the difference of the outputs of the control I5 and the speed measuring device I3:

and the resulting torque F is proportional to the product of the two inputs:

d(ubR) (ubR-cS) :F'

or dividing both sides by dc,

In order for the bomb to strike the target, the switch 25 must be closed to cause release when (from Equation '7) l T! Tf *TP There is an unavoidable time lag T1. in the response of the measuring apparatus and the release mechanism. Accordingly, the condition for closing the contacts 25 becomes:

L 5L H (lo) TH-TL Trl-TL S Tf'lTL where R is the range measured at a time Tx. before the desired release. Substituting for H (Equation 8), the condition for release is:

i L )zlz '171)' (11) Tf+TL Tf+TL S 4g Tf 'M+TL Comparison of Equations 9 and 1l shows that to cause release at the proper instant, the sensitivity control I5 must be adjusted to make C (12) "b(Tf+TL) and the torque control 23 must be adjusted so that Tf )2 (13) Tf-l- TL It is apparent from Equations l2 and 13 that Tf is the only variable in the relations for u and F', since T1. is a constant of the equipment. Therefore, u and F are functions solely of H. Thus the controls l5 and 23 may be ganged, as indicated by dash lines 21, and operated by common control means which may be manual, or may be an altitude responsive servo system such as that shown in copending U. S. patent application Serial No. 484,458, now Patent No. 2,443,748, issued June 22, 1948, flled April 24, 1943, by Royden C. Sanders, Jr. et al. and entitled Aircraft Control System. It should be noted that u and F are different functions of H. The control devices I5 and 23 must be designed with their characteristics tapered accordingly, to enable operation from a common control means.

The range and speed measuring functions may be combined in a single radio system, using but one transmitter and one receiver. Refer to Figure 3, wherein elements corresponding to those Y triangular form. Y

A with a modulating signal.

A square wave generator 31 is'coupled to the modulator 35, through an adjustable voltage dlvider |5 and a wave shaping circuit 39. The voltagedivider I5 performs the function of the range sensitivity control of Figure 1. `The wave shaping circuit converts the square wave which is applied to it to a wave of substantially symmetrical A radio receiver 4I is provided with an antenna 43, and has its output circuit connected to a voltage limiter 45. The receiver 4I is coupled also to the transmitter 33, through a line 44. The output circuit of the limiter 45 is connected t0 an averaging cycle counter circuit 41. The output of the counter 41 is applied to the upper pair of input terminals of the wattmeter I1. The output circuit of the limiter45 is connected also to a differential counter circuit 49. The differential counter 49 maybe of the type described in U. S. patent application Serial No. 524,794, led March 2, 1944, by Royden C. Sanders, Jr. et al., entitled Radio Bomb Release System, now matured to Patent No, 2,412,632, and is keyed bythe square wave generator 31 to count positive when the square wave is of positive polarity, and to count negative when the Square wave is negative. The output of the counter 49 is applied to the lower pair of input terminals of the wattmeter I1.

As in the system of Figure 2, the wattmeter I1 is coupled to the torque motor 2| by the shaft I9, which operates the switch 25. The torque control 23 comprises a variable resistor connected between the torque motor 2l and its power source 22.

In the operation of the system of Figure 3, the output of the square wave generator 31 is modified by the wave shaping circuit to provide a substan.- tially triangular wave voltage. This operates the modulator 35 to cause the frequency of the transmitter to vary accordingly, as shown by the solid line 5I in Figure 4. The mean transmitter frequency is f0. The range of variation of frequency transferred directly through the line 44 to the receiver 4I. The principal portion of the transmitter output is radiated by the antenna 33. A part of the radiated energy strikes the target and is reflected back to the receiver antenna 43.

The received signal varies in frequency like the transmitted signal, but the variations are delayed by the length of time At required for radiation to travel from the bomber to the target and back. In addition, the received signal is increased in frequency by Doppler effect, by an amount Af proportional to the rate of approach of the bomber toward its target. Thus `although the range of variation in frequency of the received signal is the same as that of the transmitted signal, the mean frequency is higher. as shown by the dash line 53 in Figure 4.

The delay of the received signal is:

aa Ato 4where o is the velocity of radiation propagation.

approximately 983,000,000 reet per second. 'me Doppler shift is:

The reilected signal combines in the receiver 4I with the transmitted signal applied through the line 44, producing a beat signal. The beat signal includes a component whose frequency is equal to the difference in frequency between the direct and reflected signals. A part of this difference is that caused by the delay At and is:

fn=gN where .Y .Y

if d! is the rate of change of transmitter frequency,

and is 20a-m4n, where fm is the modulation frequency (frequency of the square wave generator 31). Thus During modulation downsweep, the beat frequency is the sum of the range component and the speed component:

fa=2(fzfx)-f' v '-n-fo The counter circuit 41 provides an output current proportional to the average frequency of the beat signal:

il :klfu d where k1 is the sensitivity of the counter in amperes per cycle per second, and is determined by the design ofthe circuit. Since the average beat frequency is merely the range component fa, the output is 2R li=ki2(fzfi)'fu As mentioned above, the quantity (fz-f1) is controlled by the voltage divider I5, and it corresponds to the range sensitivity factor u, defined inl the description of the system of Figure 2. Since fm, v, and k1 are constants:

ii=ubR The differential counter circuit 59 provides a current comprising two alternate opposed components, i and id. During modulation upsweep, the component iu ows, and is proportional to the upsweep beat frequency fu:

i iu=k2fn where ka is a constant determined by the circuit design. During modulation downsweep, the component id flows, in the opposite direction from i0.

ia=k3fa where ka is a design constant like Ic: but of a different value.

'The averageoutput current of the counter circuit 49 is 1p-4., La-*2 since iu and id each ilow only half the time. Substituting the expressions for i and id in terms of range and speed:

Rearranging the above,

The upsweep and downswe'ep sensitivities k2 and k3 of the counter circuit 49 are adjusted so that (k2-k3) is equal to the sensitivity k1 of the counter 41, and

fuero is the factor c defined in the description of the system of Figure 2. Then and the torque provided by the wattmeter I1 is proportional to the product i1ia=ubR (ubR-cS) As in the system of Figure 2, the range sensitivity u and the torque of the motor 2| are adjusted in accordance with the altitude to cause closure of the switch 25 when the time from target equals the time of fall plus the interval Tr..

Although the systems of Figures 2 and 3 involve 'no approximations in their operation, and may be fully automatic when used in level flight, they make no provision for any vertical component V in the velocity of the bomber with respect to the target. This may be accomplished, also without approximation, by a modication of the present invention.

Refer to Figure 5, wherein elements corresponding to those of Figures 2 and 3 are designated by corresponding reference characters. The torque motor 2| of the above-described systems is replaced by a second wattmeter mechanism I1' similar to the wattmeter I1, and is connected to an altitude responsive system similar to the above-described range responsive system. The altitude channel includes areceiver 4I', like the receiver 4I but provided with an antenna 43 having a downwardly directed response pattern. The limiter 45' is identical with the limiter 45, and the counters 41 and 49 are similar to the counters 41 and 49 respectively.

The square wave generator in the present illustration includes a cam operated periodic Switch 55, driven continuously by a motor 51. The switch 55 is connected through the voltage divider I5 to a D.C. source 59, and cyclically applies the output voltage of the voltage divider I5 to a resistor 6I. Thus the voltage across the resisto-- 6I varies in square wave fashion, with an amplitude dependent on the setting of the voltage divider I5. l

The wattmeters I1 and I1 are coupled together by the shaft I9 which carries two sets of contacts 26 connected like a reversing switch between a reversible motor B3 and a power source 65. The contacts 26 are adjusted to energize the motor '63 to run only when the torque of one` v'attzxzeter exceeds that of the other, and in a direction depending upon which torque is greater.

The motor 63 is coupled to one input shaft of a differential 61. A constant speed clock motor 69 is coupled to the other input shaft of the differential 61. The output of the differential 61 drives the time from target shaft 21.

A voltage divider 1I is connected across the D.C. source 59, and its output is applied to Aone winding of a differential relay 13. The other winding of the relay 13 is connected to the output of the diierential counter 49 of the altitude channel. A

In the operation of the system of Figure 5, as in that of Figure 3, the counter 41 provides an output i1=ubR, and the diierential counter 49 provides an output ia=ubR-cS. Now suppose that the shaft 21 is at a position such that the voltage divider I5 is-adjusted to make L WT Then the torque F of the wattmeter I1 will be proportional to:

R R MTG-S) A pair of resistors 15 and 11 are included in the connections from the counters 41 and 49 to the wattmeter I1 for a purpose to be described hereinafter. These do not aiect the present analysis, since they merely alter the constant factor of proportionality of F to R R a-{r-S) The altitude counter 41' operates like the range counter 41, providing an output i1' bearing the same sort of relationship to the altitude as the current ii bears to the range, i. e.

and

The torque' of the wattmeter I1 is thus:

H H Padri) The overall sensitivities of the range and altitude channels are made to correspond so that F=F when which is Equation 5.

Thus so long as the shaft 21 is in a position corresponding to the present time from target T, F=F and the motor 63 is deenergized.

The clock motor 69, through the differential 61, drives the shaft 21 at a constant speed corresponding to the passage of time. Thus, if the shaft 21 is set initially` at the position corresponding to the then time from target, the clock motor 69 will rotate it to continue to correspond to the predicted time from target. If the shaft 21 does not correspond. to the time from target, the torque of one of the wattmeters I1 and I1 will overcome that of the other, energizing the motor 63 and causing it to rotate in the direction required to set the shaft 21, by way of the differential 81, to its proper position. With this arrangement, temporary failure of the range signal as a result of fading will not prevent operation or'cause serious error, since the equipment will. d continue to operate as a clock indicating time from'target, on the basis of the last available data.

Since the operation of the counters and the servo is subject to an unavoidable time lag Trl, the servo will not set the outputl of potentiometer I to present time from target T as determined by present range R but rather to past time from target T-I-Tr.1 as determined by radar measurement of the corresponding past range R'. By suitable setting with respect to the resistancerotation characteristic of potentiometer I5, the servo shaft position may nevertheless represent present time from target T.

A plane which is now at altitude H and descending at constant vertical velocity V and will cross over the target after a time T, will cross at an altitude Hu where Ho=H-VT Altitude, vertical speed and time of fall of a bomb released from this plane are always related by where H1- is the altitude at release. If the bomb is to hit the target, its time of` fall must equal the time from target at release, and

The equipment measures altitude H at time from y target T+TL1, and actual release should occur at time T-TL2, where T is the time indicated by the position of the shaft 21 when the actuating condition for the release relay is fullled and T1.2 is the time lag in operation of the release relay and release mechanism. Use of these `quantities in `Equations 16, 17 and 18 in comthat its to ow in the lower coil of the relay 13, where TL., is the time interval required for operation of the relay 13 and the bomb release mechanism. The proportionality of in to T is made such that z=i1 when 10 8| is applied to both input circuits of the Wattmeter movement I1 through resistors 83 and 85 respectively.

The resistors 83 and 85, as well as the previously mentioned resistors 15 and 11, are much higher in resistance than the respective input circuits of the wattmeter I1. With this arrangement, the current in the upper input circuit of the wattmeter I1 is proportional to the sum of the output i1 of the counter 41 and a current i: determined by the setting of the voltage divider 8l and the position of the voltage divider I5. Similarly, the input to the lower terminals of the wattmeter I1 is proportional to the sum of the output is of the counter 49, and the current is. The torque of the Wattmeter I1 is thus increased or decreased by an amount such as to cause the bomb to fall short or fall beyond the target by a distance depending only upon the -adjustment of the voltage divider 8|, and independent of the altitude or time from target.

Throughout the above description, it has been assumed that the effects of air resistance on the trajectory 0I. the bomb are negligible. However, at high bomber speeds and moderate altitudes, the bomb does not remain directly under the aircraft during its fall, but trails, or falls behind the bomber. The trail is a function substantially only of the air speed of the bomber and the ballistic characteristics of the bomb itself. Thus, for a given type of bomb and operating air speed, a certain trail angle should be included in the computations for bomb release.

In the system of Figure', a voltage divider 81 is connected across the output of the altitude counter 41', and is coupled to the upper and lower terminals of the wattmeter I1 through resistors 89 and 9| respectively. These resistors, like the resistors 83 and 85, are much higher in resistance than the wattmeter itself. Thus the effect of the voltage divider 81 and the resistors 89 and 9I is to add a current i4, 'proportional to the output iv of the counter 41' to the currents i1 and ie respectively in the wattmeter I1. This alters the torque of the wattmeter I1 in accordance with the adjustment of the voltage divider 81, as if the range R were decreased by an amount H sin pl, and thus eiects compensation for trail.

I claim as my invention:

1. In a radio bomb release system including means responsive to the slant range of a target from a bomber craft to provide an electrical output proportional to said slant range, and means responsive to the slant speed of said bomber with respect to said target to provide a second electrical output proportional to said slant speed, means for adjusting the proportionality of said rst output to said slant range, means responsive to said adjusted rst output and to said second output to produce a force proportional to the product of said adjusted first output by the difference of said adjusted iirst output and said second output, means providing a second force, and means responsive to equality of the magnitudes of said rst and second forces to effect bomb release.

2. In a radio bomb release system including means responsive to the slant range of a target from a bomber craft to provide an electrical output proportional to said slant range, and means responsive to the slant speed of said bomber with respect to said target to provide a second electrical output proprotional to said slant speed, means for adjusting the proportionality of said rst output to said slant range, means responsive to said adillted rst output and to said second out- 1 1 put to produce a force proportional to the product of said adjusted rst output by the difference of said adjusted rst output and said second output, and means responsive to the attainment by said force of a predetermined magnitude to effect bomb release.

3. In a radio bomb release system including means responsive to the slant range of a target from a bomber craft to provide an electrical output proportional to said slant range, and means responsive to the slant speed of said bomber with respect to said target to provide a second electrical output proportional to said slant speed, means for adjusting the proportionality of said rst output to said slant range, means responsive to said adjusted first output and to said second output to produce a force proportional to the product of said adjusted first output by the difference 'of said adjusted first output and said second output, means providing an adjustable second force, and means responsive to the attainment of equality by said forces to effect bomb release.

4. In a radio bomb release system including means responsive to the slant range of a target from a bomber craft to provide an electrical output proportional to said slant range, and means responsive to the slant speed of said bomber with respect to said target to provide a second electrical output proportional to said slant speed, means for adjusting the proportionality of said first output to said slant range in accordance with a predetermined function of the altitude of said craft, means responsive to said adjusted rst output and to said second output to produce a force proportional to the product of. said adjusted rst output by the difference of said adjusted first output and said second output, means providing a second force which is a second predetermined function of the altitude of said craft, and means responsive to the attainment of equality by said forces to effect bomb release.

of said transmitter means to provide an `output differing from that of said averaging counter by an amount proportional to said slant speed, means for adjusting the rate of change of frequency of said transmitter means to adjust the proportionality to said slant range of said counter outputs, means responsive to both of said counter outputs to produce a force proportional to their product, and means responsive to the attainment by said force of a predetermined value to effect bomb release.

'7. A bomb release system including frequency modulated radio transmitter means, radio receiver means providing a beat signal varying cyclically in frequency about a mean frequency Aproportional to the slant range of a bomber craft from a selected target, throughout a band Whose width is proportional to the slant speed of said bomber with respect to said target, averaging counter means responsive to said beat signal to provide van output proportional to said slant range, differential counter means responsive differentially to said beat signal during increase and decrease offrequency of said transmitter means to provide an output differing from that of said averaging counter by an amount proportional to said slant speed, means for adjusting the rate of change of frequency of said transmitter means to adjust the proportionality to said slant range of said counter outputs, and means responsive to both of said counter outputs to produce a force proportional to their product; further radio receiver means providing a second beat signal, corresponding to the altitude and vertical velocity of said bomber like said first beat signal corresponds to said range and said speed; further averaging counter means respon- 5.'In a radio bomb release system including means responsive to the slant range of a target from a bomber craft to provide an electrical output proportional to said slant range, and means responsive to the slant speed of said bomber with respect to said target to provide a second electrical output proportional to said slant speed, means-for adjusting in substantially reciprocal relation to the time from target at which a bomb is to be released the proportionality of said rst output to said slant range, means responsive to said adjusted first output and to said second output to produce a force proportional to the product of said adjusted rst output by the difference of said adjusted first output and said second output, means providing a second force adjustable substantially in proportion to the time from target at which a bomb is to be released, and means responsive to the attainment of a predetermined ,difference between said forces to effect bomb release.

6. A bomb release system including frequency modulated radio transmitter means and receiver means providing a beat signal varying cyclically in frequency about a mean frequency proportional to the slant range of a bomber craft from a selected target, throughout a band whose width is proportional to the slant speed of said bomber with respect to said target, averaging counter means responsive to said beat signal to provide an output proportional to said slant range, counter means responsive differentially to said beat signal during increase and decrease of f reqlleflcy sive to said second beat signal to provide an output proportional to said altitude, further differential counter means responsive to 'said second beat signal to provide an output differing from that of said further averaging counter by an amount proportional to said vertical velocity, means responsive to the outputs of both of said further counters to produce a second force proportional to their product, and servo means including an output shaft and responsive to the dierence between said first and second forces to control said means for adjusting said rate of change of frequency of said transmitter and thereby maintain said output shaft at a position corresponding to the time interval required for said craft to reach said target.

8. A bomb release system including frequency modulated radio transmitter means, radio receiver means providing a beat signal varying cyclically in frequency about a mean frequency proportional to the slant range of a bomber craft from a selected target, throughout a band Whose width is proportional to the slant speed of said bomber with respect to said target, averaging counter means responsive to said beat signal to provide an output proportional to said slant range, differential counter means responsive differentially to said beat signal during increase and decrease of frequency of said transmitter means to provide an output differing from that of said averaging counter by an amount proportional to said slant speed, means for adjusting the rate of change of frequency of said transmitter vmeans to adjust the proportionality to said slant range of said counter outputs, and means responsive to both of said counter outputs to produce a force proportional to their product; further radio receiver means providing a second 13 beat signal, corresponding to the altitude and vertical velocity of said bomber like said first beat signal corresponds to said range and said speed; further averaging counter means responsive to said second beat signal to provide an output proportional to said altitude, further differential counter means responsive to said second beat signal to provide lan output, differing from that of said further averaging counter by an amount proportional to said vertical velocity. means responsive to the outputs of both of said further counters to produce a second force proportional to their product, servo means including an output shaft and responsive to the attainment of equality by said first and second forces to control said means for adjusting said rate of change of frequency of said transmitter means and thereby maintain said output shaft at a position corresponding to the time interval ,required for said craft to reach said target; means responsive to the position of said output shaft to provide an output current substantially propoxtional to said time interval, and relay means responsive diiierentially to said last-mentioned current and to the output current of said second averaging counter to eiect bomb release Vupon the attainment .of a predetermined magnitude relationship between said two last-mentioned currents.

9. In combination in a bomb release system, means responsive to slant range and slant speed.

respectively providing voltages oi' magnitudes proportional respectively to slant range and slant speed, a wattmeter mechanism including two input circuits, means applying said range-proling the value of said predetermined torque in ,Y

accordance with said altitude.

-DAVID G. C. LUCK.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 2,268,587 Guanella Jan. 6, 1942 2,412,631 Rice Dec. 17, 1946 2,412,632 Sanders et al Dec. 17, 1946 2,416,223 Sanders Feb. 18, 1947 2,438,112 Darlington Mar. 23, 1948 Fyler July 6, 1948 

